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Fall 2000, Vol. 10, No. 3 Institute for Rock Magnetism

Ytterby Mine, near Stockholm, where the rare earths (oxides of the lanthanide elements) were first discovered.

Wherefore Gadolinium? Magnetism of the Rare Earths Mike Jackson IRM

In 1794, the Finnish chemist Johan Gadolin, investigating an unusual mineral specimen from Sweden, discovered what he believed was a new element. Although he could not have imagined it, his discovery provided the basis for the best susceptometer calibration standards available today, as well as the strongest permanent magnets. But that is getting ahead of the story. What’s in a Name? Chemists of classical Greece defined an earth as a substance that could not be broken down by the heating methods then available, and that classification scheme was still in use in the 18th

century. (Early in the 19th century Sir Humphrey Davy showed that the earths were not in fact elements, but oxides of metallic elements.) Gadolin classified his new “element” as an earth, and named it Ytterbia, for the village of Ytterby where the sample was discovered, near Stockholm. The same mineral specimen yielded a second new earth in 1803, named Ceria after the asteriod Ceres, which had been discovered in 1801. (Ceres, you may recall, provided Gauss with one of his early mathematical/ scientific triumphs, when he applied his method of least squares to establish accurate orbital parameters from a very limited set of observations, and thereby enabled a quick re-discovery of the asteroid after its initial diappearance behind the sun.) Subsequent work showed that Ytterbia and Ceria each actually contained several different rare earths. In the mid 1800s Carl-Gustav Mosander isolated two new elements in Ytterbium, to which he gave the derivative names Terbium and Erbium, and two others in

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Cerium, which he named Didymium (from the Greek for ‘twin’) and Lanthanum (from the Greek lanthanein, ‘to lie hidden’). These in turn were further resolved into their actual elemental constituents with the advent of absorption and emission spectroscopy around 1860. Gadolin’s Ytterbia was ultimately found to contain most of the heavy rare earth elements (REEs): Holmium (#67, named for Stockholm), Thulium (#69, for Thule, an early name for Scandinavia), Dysprosium (#66, from the Greek dysprositos, ‘hard to get at’), Lutetium (#71, for Lutetia or Lutece, the ancient name for Paris, where a number of these elements were first isolated), and of course Gadolinium (#64) and Ytterbium (#70). From Mosander’s Didymium came the light REEs Samarium (#62, for the mineral Samarskite, named in turn for a Russian mining official called Samarski), Praesodymium (#59, ‘green twin’), Neodymium (#60, ‘new twin’), and Europium (#63). Rare earths are not as rare as originally thought. The Earth’s crust contains more cerium (the most abundant REE) than tin, cobalt or beryllium; lanthanum and neodymium are more than three times as abundant as lead. The least abundant of the stable rare earth elements are thulium and lutetium, but they are still 100 times more abundant than gold. Prometheum (#61, for Prometheus, who stole fire from the gods) was not isolated until the 20th century; it has a half-life of less than 20 years for all known isotopes, and essentially does not occur in nature (at least not in the earth’s crust; it has been identified in the spectra of stars such as HR465 in Andromeda.) Ferromagnetism in Rare-Earth Metals Ferromagnetism is a relatively rare property among the elements, occurring only in the transition metals Fe, Ni, and Co, and in the lanthanides (REEs). Neutron-diffraction studies in the 1960’s showed that magnetic ordering in the rare earths is generally far more interesting (i.e., complicated) than the simple collinear ferromagnetism of substances like iron, in which all the atomic magnetic moments are resolutely and monotonously parallel. The REEs collectively exhibit an astonishing bestiary of magnetically-ordered states, and even individual REEs undergo chameleon-like tranformations, ordering in a variety of REEs continued on page 6...

Visiting Fellows’ Reports

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This project is part of my PhD studies under the supervision of Prof. N. Petersen. The objective was to determine saturation magnetization MS at 0 K and the mechanism controlling magnetic stability (or coercivity HC) of natural titanomaghemites in ocean basalts. In total, 13 samples of ocean basalt of different age and titanomaghemite oxidation state were studied (3 samples with low, 7 samples with medium, and 3 samples with high oxidation state titanomaghemite). Magnetic hysteresis loops were measured at temperatures ranging from 300 K to 10K. Additionally, thermomagnetic curves at different magnetic fields were measured in this temperature range. Magnetic Force Microscope (MFM) techniques were used to image magnetic structures in titanomaghemite grains of two selected samples. The samples were selected from a set of 80 samples I had studied previously in Munich that show high MS and low HC values for 0-8 Ma (weakly oxidized) and 40 to 120 Ma (highly oxidized) old basalts, but low MS and high HC for 8 – 40 Ma old or medium oxidized ocean basalts. Low MS values for the latter samples can be explained by a model with differential oxidation of Fe in the octahedral and tetrahedral sites of the spinel lattice. Determination of MS at 0 K allows testing of this hypothesis, as the number of Fe cations in octahedral sites minus the number in tetrahedal sites can then directly be calculated, if the content of titanomaghemite in the samples is determined microscopically. The high magnetic stability for these samples can be explained by internal stress arising from the oxidation process. It is an interesting observation that these samples often show ratios of saturation remanence to saturation magnetization MRS/MS exceeding 0.5, apparently ruling out uniaxial anisotropy and (magnetostrictive) stress control. However, MRS/ MS can be erroneously overestimated, if the maximum field applied does not saturate the sample. For all samples, hysteresis loops between 10 K and 450 K were measured with the µ-VSM (maximum field 1.7 T). The 7 samples (8 – 40 Ma old) with high coercivity were also measured with the MPMS between 300 K and 10 K. The maximum field of this instrument (5 T)

Using the MPMS for hysteresis measurements is rather uncommon and therefore I want to concentrate on the results of these measurements. The unique advantage of these instruments are the high magnetic fields which can be applied. For best results, the instrument had to be operated in “no overshoot mode” rather than in the much faster “hysteresis mode”. Although basalts give a strong magnetic signal, and the MPMS is a very sensitive instrument, good results were obtained only for samples >5 mg. Samples >20 mg were excellent and samples